1. Field of the Invention
The present invention relates to a magnetic resonance imaging method, and more particularly to a method for reconstructing an image from an echo planar imaging (EPI) sequence.
2. Description of the Prior Art
An EPI imaging sequence is a fast magnetic resonance imaging sequence which only needs singe-shot pulses or multi-shot pulses to obtain a sequence of complete images. It generates a series of gradient echoes by means of alternating changes of the gradient of a magnetic field, and an image of the excited plane is obtained by performing Fourier transformation to the acquired k-space data.
In a conventional EPI sequence, the phase encoding gradient and the frequency encoding gradient of the sequence are illustrated with reference numbers 1 and 3 respectively in FIG. 1. Due to imperfections in the hardware system, such as eddy currents, delay in data acquisition, the nonuniformity of the basic magnetic field, Maxell effects, inconsistent magnetization, etc., the readout data under positive and negative frequency encoding gradients are not completely consistent. For example, there are cases such as the echo centers are inconsistent, the spacing between each line of the k-space is not equal, etc. Due to the mismatch of time or the phase differences between the odd and even echoes in an echo sequence, a low intensity overlapping image appears on a normal image after its reconstruction at a position half way offsetting an actual field of view (FOV), that is, the appearance of N/2 artifacts (also referred to as Nyqusit artifacts). In addition, due to the non-uniformity of the main magnetic field and the inconsistent magnetization, the odd and even echo signals cancel each other, leading to a signal loss in the reconstructed image.
In order to eliminate N/2 artifacts as much as possible, some correction methods, such as phase correction methods, unipolar readout gradient methods, post-processing methods, over-sampling methods, etc., are utilized in the prior art.
One of the phase correction methods is a one-dimensional phase correction method, in which before reading the image data, several echo signals without phase encoding, such as the echo signal 4 in FIG. 1, are sampled; and according to the first and the second echo signals, the position difference between their echo centers is calculated and used as a correction value to correct the collected image data, that is, to correct the echo center differences between all the odd and even lines. Since not all the echo center deviations between the odd and even lines are equal to the calculated correction value, this method can remove N/2 artifacts to a certain level and reduces the image signal loss, but it cannot eliminate all the artifacts. Furthermore, this method is unstable, especially in a medium or low permanent-magnet field. Due to the poor uniformity of the magnetic field, the relatively large gradient eddy currents and the relatively low magnetic field, the N/2 artifacts are still prominent after the phase correction, and sometimes they become even more prominent than those without the phase correction.
Another one of the phase correction methods is a two-dimensional phase correction method, in which, likewise before reading the image data, a series of echo signals with phase encoding are sampled and are then used to correct the actual image data. Although this method can solve the problem of the N/2 artifacts effectively, the rapidity of the EPI sequence is lost because of the prolonged time for sequence acquisition.
In the unipolar readout gradient method, data are collected only during the activation of a readout gradient of one polarity, so the method can avoid the problem of N/2 artifacts completely, but it cannot correct the image deformation and it wastes the scanning time. Furthermore, due to the extended echo intervals, it leads to a decrease of the equivalent sampling bandwidth in the phase encoding direction, and an increase of image deformation and distortion.
Post-processing methods can reduce the N/2 artifacts to a certain degree, but cannot avoid this problem completely. Moreover, because such methods ensue in the post-processing of already-acquired data, an image processed and corrected by this method suffers a significant loss in image information and also its reconstruction time is quite long.
An over-sampling method is disclosed in U.S. Pat. No. 6,320,238 that performs over-sampling in the phase encoding direction of an EPI imaging sequence to enlarge the FOV in the phase direction, so as to correspondingly shift the N/2 artifacts out of the displayed FOV, and to achieve the goal of eliminating the N/2 artifacts. Compared with above-mentioned methods, the over-sampling method is the simplest and the most effective, but this method cannot overcome the problem of image signal loss.